China has promised four billion yuan (US$588 million) in aid to Cambodia, the Southeast Asian country's Prime Minister Hun Sen said Tuesday on a visit to Beijing.

The largest investor in Cambodia, China has pumped billions of yuan into the economy and has written off debt while sweeping aside questions over Phnom Penh's abysmal rights record.

The four-billion-yuan grant will run from 2019 to 2021, Hun Sen said in a post on his official Facebook page, accompanied by pictures of his meeting with Chinese President Xi Jinping from the day before.

Xi has also pledged to import 400,000 tonnes of rice from Cambodia this year, increase bilateral trade to US$10 billion by 2023 and encourage more Chinese investment into the country, the prime minister's post said.

The pledge comes just days after the European Union (EU) resumed tariffs on rice imports from Cambodia, following intensive lobbying by Italy, who said cheap imports were damaging its farmers.

"I am grateful to China for its long-standing support and valuable help to Cambodia, and for China's friendship...especially with the royal family," Hun Sen said at the meeting, according to the official Xinhua news agency.

Hun Sen was welcomed at the ornate Great Hall of The People by Premier Li Keqiang on Tuesday as part of his three-day visit.

After closed-door talks, the two men witnessed the signing of six agreements, including for increased cooperation on Beijing's Belt and Road infrastructure initiative.

"The major points of our discussions are in the political, economic and security spheres," head of the Chinese foreign ministry's Asia department Wu Jianghao told reporters.

Bilateral trade reached US$4.7 billion from January to August 2018, up 23.8 percent year-on-year, China's Ministry of Commerce said in November.

Long, volatile relationship

Hun Sen's visit comes amid speculation Beijing is seeking support to build a naval base off the Cambodian coast, claims the premier has vehemently denied.

"The issue of national defence is sensitive," said Shi Yinhong, a researcher at Renmin University's School of International Relations.

"If the relationship is too deep, it may complicate the relationship between China and some of Cambodia's neighbours," he said, adding that Beijing has chosen to be cautious.

China has had a long and volatile history with its Southeast Asian neighbour. After backing the Khmer Rouge, it fell out of favour when the Pol Pot regime was deposed.

Ties have since been revived in the past decade under the government of Hun Sen, who marked 34 years in power just over a week ago.

Cambodia has proved a reliable ally among the Association of Southeast Asian Nations (ASEAN) bloc regarding disputes with China over its activities in the South China Sea.

Beijing claims virtually the entire waterway, reclaiming reefs and building military installations, moves which have angered rival claimants including Malaysia, Vietnam and the Philippines.

China has also backed Cambodia on sensitive issues, including a controversial election held in July after Hun Sen's government banned the main opposition party from taking part.

The mutual support comes as United States influence declines – Phnom Penh has accused Washington of conspiring with an opposition leader to overthrow the government and suspended military exchanges with the country. - AFP